1. Field Of The Invention
This invention relates to electrodes for use in electrolysis of aqueous solutions of metal halides, such as alkali metal or alkaline earth metal halides, and especially relates to electrodes suitable for use in electrolysis of dilute brine, such as sea water at low temperature, and also to a method of making the same.
2. Description Of The Prior Art
Up until the present, an electrolytic apparatus, which electrolyzes dilute brine, usually an aqueous solution with a halide concentration of about 15% by weight or less, such as sea water, and evolves chlorine gas at the anode side, has been used for the prevention of the adhesion of marine life to marine structures and for water treatment at swimming pools, waterworks and sewage treatment plants.
In these electrolytic processes using an electrolytic apparatus which has no diaphragm, chlorine gas is evolved at the anode side and hypochlorite ion is produced by reaction of chlorine and hydroxyl ion. The hypochlorite ion produced is useful for sterilization purposes and for bleaching, for instance. Because operation of such an electrolytic apparatus is continuous for a long time, desirably at good efficiency and in a stable manner, and is usually outdoors, the anodes used must have especially high durability and, at the same time, the characteristics of the electrode should be maintained.
Namely, in an electrolysis such as a sea water electrolysis where the electrolytic conditions differ from a chlorine-alkali metal hydroxide electrolysis in concentrated brine, e.g., brine containing about 20% by weight to a saturated amount of halide, the electrolytic conditions, such as the concentration and the temperature of the electrolyte, are not fixed and the concentration of sodium chloride is quite low, ordinarily about 3% by weight, and the temperature of the sea water goes below 20.degree. C. Therefore, requirements such as sufficiently high chlorine evolution efficiency and durability must be met under these conditions. Heretofore, various kinds of electrodes for brine electrolysis with a coating of which the main components are platinum group metals, such as ruthenium, or oxides thereof, on an anti-corrosive substrate, such as titanium, are well known, for example, as disclosed in Japanese Patent Publication No. 3954/1973 (corresponding to U.S. Pat. No. 3,711,385), etc.
It can be said that these known electrodes described above are designed to evolve chlorine gas with good current efficiency, and efforts are made to achieve a low chlorine evolution potential and a large difference between oxygen and chlorine evolution potentials. These electrodes are considered sufficient for use in concentrated brine electrolysis at relatively high temperature, e.g., about 60.degree. to 105.degree. C., usually around 90.degree. C., such as in chlorine-alkali metal hydroxide electrolysis, but they are not always advantageous for use in dilute brine electrolysis at low temperature, e.g., at below about 25.degree. C., as in sea water electrolysis.
On the other hand, known electrodes for use in dilute brine electrolysis such as sea water electrolysis are described in Japanese Patent Applications Nos. (OPI) 58075/1977 and 13298/1975 (corresponding to U.S. Pat. No. 3,917,518) (the term Japanese Patent Application (OPI), as used herein, means a published but unexamined Japanese patent application).
Japanese Patent Application No. (OPI) 58075/1977 describes an electrode having a coating of which the principal component is palladium oxide on an electroconductive substrate. This electrode can be expected to have good chlorine evolution efficiency in electrolytic processes at relatively higher temperatures, but the corrosion resistance of this electrode is unsatisfactory at low temperatures, especially at lower than 20.degree. C., and problems occur with this electrode since complicated procedures are involved in its manufacture because palladium metal must be completely absent from the coating of the electrode.
Japanese Patent Application No. (OPI) 13297/1975 discloses an electrode for use in a process of producing hypochlorite which has a coating of oxides of tin, antimony, a platinum group metal and a valve metal, such as titanium, on an electroconductive substrate. This electrode would appear to be useful in electrolyzing sea water at relatively low temperatures. However, antimony oxide is an essential component of the coating, and since antimony oxide vaporizes easily during the electrode coating procedure, the yield is not good. As a result, it is difficult to obtain an electrode of the desired composition in a reliable and stable manner.